Filler-containing polyurethanes which have a high water absorbability are described in German Offenlegungsschrift 3,151,925 as being in the form of abrasive-containing, aqueous polyurethane gels (hydrogels). In that publication, from 20 to 80% by weight of abrasive (such as, for example, aluminum oxide, cerium oxide, tungsten dioxide, boron carbide, silicon carbide, asbestos powder, graphite, glass microspheres, mica or short fibers), and optionally fungicides, dyes or coloring pigments are used, to produce elastic abrasive gels derived from polyurethane(ureas). The compositions are prepared by reacting hydrophilic isocyanate-terminated prepolymers (i.e., those derived from polyoxyalkylene ethers containing 30% or more by weight of oxyethylene groups) with water. The water absorbability of the gels is based on the use of hydrophilic (i.e., oxyethylene group-containing) polyether polyols as the starting materials.
German Offenlegungsschriften 2,347,299 and 2,521,265 describe water-swollen polyurethane(urea) gels which are homogeneous or are blown by CO.sub.2 in the manner of a foam, and which may contain up to 50% by volume of fillers (such as silicates, silicas, aluminum oxides, tin oxide, antimony trioxide, titanium dioxide, graphite, graphitic carbon, retort carbon, carbon black, pulverulous types of cement, coloring pigments, fibers and cellulose powder) in addition to surfactants or nutrients. In these publications as well, the water absorbability of the gels is essentially based on the use of hydrophilic polyether polyols which contain 40% by weight or more of oxyethylene sequences.
German Offenlegungsschrift 3,103,499 describes substantially anhydrous polyurethane gel compositions using polyols as the dispersing agent. The gel compositions may contain active substances and dyes, pigments, fibers, inorganic fillers, metal powders, active carbon, cellulose powder and silicas. Polyol-containing gels of this type are unsuitable for use in an aqueous environment, because they release a large amount of the dispersed polyols in aqueous suspension.
It is also known to embed cells which are capable of growth in polyurethane hydrogels, see, for example Tanaka et al, European Journal of Applied Microbiology and Biotechnology, 7, (1979) from page 351. German Offenlegungsschrift 2,929,872 also describes a process for the production of hydrophilic, gel-like or foamed biocatalysts which have a high charge of enzymatically active substance by the polymer inclusion of complete cells, fragments of cells or enzymes, by mixing an aqueous suspension of the enzyme-active substance with hydrophilic polyisocyanates to form an enzyme-active, hydrophilic polyurethane network in block- or bead form. Other publications pertaining to the prior art are noted on page 7 of the Offenlegungsschrift.
In the prior art polyurethane gels, hydrophilic polyurethanes based on polyethers must be synthesized using ethers containing large amounts of oxyethylene segments in order to achieve a satisfactory water absorbability. Problems of reactivity of the hydrophilic polyether polyols (which usually exhibit high activities), and problems in the mechanical gel strength when highly hydrophilic polyether polyols are used often arise. In addition, such compositions are relatively expensive.